The present invention relates generally to the field of gypsum boards, and more particularly to water-resistant gypsum boards and methods for their manufacture.
Gypsum boards are commonly used as the exterior sheathing panels in building construction. Because sheathing panels may be exposed to water or high humidity, gypsum boards for these applications should be water-resistant to reduce or eliminate: loss of board strength, microbiological growth, and delamination of facings. Accordingly, many attempts have been made to improve the water-resistance of the set gypsum that forms the gypsum boards, which has a tendency to absorb water.
These attempts have included the incorporation of water-resistant additives such as metallic soaps, asphalts, siloxanes, and resins into the gypsum slurry, as well as coating finished gypsum boards with water-resistant films or coatings. For example, one siloxane additive that has been used to improve the water-resistance of gypsum products is polymethylhydrogensiloxane (PMHS). Generally, a small amount of siloxane is added to the gypsum slurry before the board is formed and dried. It has been observed, however, that the siloxane frequently does not completely cure in a reasonable period of time or never completely cures. In these cases, the water resistance does not develop to a satisfactory level and the siloxane can be released as a VOC from the finished board.
It has been determined that for such siloxane additives to cure properly during the gypsum board manufacture process, they may be thermally cured or cured with a curing agent. For example, curing agents may include siliconates, such as potassium methyl siliconate or sodium methyl siliconate. When a thermal siloxane curing process only is used, it is critical that the temperature of the slurry reach curing temperature. Otherwise, the siloxane curing reaction may not be completed for sometime after manufacture. Curing agents must be provided in amounts that do not cure the siloxane too rapidly and do not interfere with the rehydration and setting of the gypsum during the board manufacturing process.
Accordingly, it would be desirable to provide improved water-resistant gypsum boards utilizing siloxanes, and methods and compositions for their manufacture.